Handbook of Modern Coating Technologies

Near-field microwave diagnostics of materials and media

Near-field microwave diagnostics is a direct nondestructive method providing information on the surface and nearsurface properties of various media. It is based on recording the microwave fraction of an impact in the probe near-field, thereby allowing a significant improvement in the spatial resolution and overcoming the diffraction limit for given frequencies [176—180].
Localization of a microwave signal in the near-field is accomplished using devices whose design and operation principles are considered below. The main factors determining the spatial resolution and accuracy of measurements (signal/noise ratio) are the design of the measuring device, properties of the material being studied, probe size, and distance between the probe and the sample surface. The data obtained are treated by elaborate methods of mathematical physics and numerical analysis, with the involvement of experimental techni-ques for the diagnostics of materials in the microwave range [176—180].
Fig. 5—18 presents the classification of ultramicroscopic (nanoscale) methods, highlight¬ing near-field scanning, micro-wave, and microprobe microscopies. High resolution is achieved by virtue of a special design of measuring transducers (MTs). Classification of MTs for microwave diagnostics as presented in Fig. 5—19 categorizes them into two main classes, resonator and wave systems, which are, in turn, subdivided into subclasses. The following notations are specified in Fig. 5—19: IDR-internal dielectric resonator (DR), MDR-microwave
Microwave
generator

FIGURE 5-18 Modern microdiagnostic techniques. 

FIGURE 5-19 Classification of MTs for microwave diagnostics. MTs, Measuring transducers.

DR, ODR-open DR, MS-translation stage, NFI-near-field interaction, SE-sensitive element, CMEB-coaxial measuring equipment, and COR-cylindrical open resonator.
Near-field microwave diagnostics is extensively invoked to study the surface of dielectric and semiconducting films, to map material permittivity, to detect minor defects and irregu-larities, and to analyze nonlinear characteristics. It finds wide application in biology and medicine. The application of microwave waves in biological research has important advan-tages over the use of visible and IR wave-lengths due to their high penetrating capacity (from a few millimeters to decimeters). It permits exploring not only superficial layers but also deep-lying ones (at the expense of the respective loss of spatial resolution on the sur¬face). This property is used to analyze and visualize structures of such biological objects as neoplasms [177,179].
Equally promising are microwave studies in semiconductor micro- and nanoelectronics because they provide multi-parameter information about surface and near-surface layers. An important field for the application of microwave diagnostics is microwave microscopy in nanotechnologies, which is currently used in the fabrication of high-temperature supercon-ductors, visualization of surface conductivity distribution, local measurement of nonlinear microwave response, etc. 
Sample
Decoupler
FIGURE 5-20 (A) Different types of measuring transducers. (B) Schematic layout of a microwave microscope. (C and D) Geometric diagrams of RMTs for a microwave microscope [47]. RMT, Resonator-type measuring transducer.
Fig. 5—20A depicts different types of measurement converters, namely, coaxial MT, com-posite resonator-type MT (RMT) with an enhanced Q factor, and stripline MT [180]. Fig. 5—20B presents the schematic layout of a microwave microscope. 
The main advantage of microwave near-field microscopy is that it is a multifunctional technique giving the possibility of additional manipulation of the specimen, for example, by applying constant electric field and magnetic field, an additional microwave field, mechani¬cal and force fields, etc.;what is especially important is that it allows comprehensively study¬ing the properties of surface layers in the microwave range.
The physical principles of the scanning microwave microscopy (SMWM) for semiconduc-tors were developed in Ref. [177] in conjunction with the general concept of enhancing its spatial resolution (at the 100-nm level) and sensitivity (multiparametricity). The concept con-sisted in the maximum spatial localization of the probing microwave field energy in the elec-tric constituent of the coaxial resonator microprobe, which is normal to the test object; it also consisted in the formation of scanning signals with a wide application of modulation principles and their additional information processing relying on modern RMT design facili-ties for SMWM with the separation of microwave field-accumulating region and that of emis-sion into the microprobe. Fig. 5—20C shows geometric diagrams of RMTs for the case of local changes in sample characteristics.
The same authors achieved a spatial resolution of 1 mm [177—180]. Analysis of para-meters of certain microwave microscopes [179] shows that their reconstruction (modifica¬tion) may lead to a 10-fold enhancement of spatial resolution, that is, at a level of 100 nm or possibly better.
Fig. 5—21A displays a photograph of the experimental prototype of the scanning micro-wave microscope, and Fig. 5—21B presents an image taken by two-dimensional scanning of the profile of a microcircuit fragment (color image online). Table 5.3 illustrates SMWM appli-cations in nanotechnologies.